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March 31, 1964 F. W. BEALL METHOD OF DECARBURIZING STEEL COIL Filed Dec.4, 1961 Emperafure f) 2 Sheets-Sheet 2 7400 Reduc'me To Iron I300 Roxldizine +0 Iron r200 Fig. 4

INVENTOR.

FRANCIS W. BEALL,

Awausvs United States Patent 3,127,289 METHOD OF DECARBURIZING STEELCGIL francis W. Beall, deceased, late of Middletown, Olno, by

Edythe E. Beall, exeeutrix, Middletown, Ohio, assignor to Armco SteelCorporation, Middletown, OlllO, a corporation of Ohio Filed Dec. 4,1961, Ser. No. 157,322 4 Claims. (Cl. 148-46) The invention relates tothe decarburization of steel coil and more particularly todecarburization effected in an opened coil annealing operation.

The decarburizing anneal of steel to remove the strains of cold rollingas well as to remove the carbon from the strip is a known expedient, andhitherto various procedures have been proposed for enhancing theeffectiveness of the decarburizing operation. For example, it has beenrecognized for some time that water vapor is a particularly elfectivedecarburizing substance, and this is particularly true if water vapor ispresent in controlled amounts in gases containing hydrogen. Theatmospheres usually employed are the commercially available andrelatively inexpensive gases such as HNX i -8% H balance N or DX (11.5%H CO 10% CO, and the balance N Dissociated ammonia also may be employed.If HNX or DX gas are employed they are normally admixed with sulficientwater vapor and hydrogen or dissociated ammonia to establish a dew pointin the range of +90 to +130 F. Ideally, the most efiective gas isstraight hydrogen made as wet as possible while maintaining anon-oxidizing potential; however gas mixtures of various hydrogencontents may be used with suitable dew points to attain comparableresults. In any event, in all cases the major decarburization reactionis: C+H O==CO+H and ideally, the H /H O ratio should be maintained inequilibrium so that the atmosphere in the annealing furnace will bedecarburizing and yet nonoxidizing at all times.

Considerable difficulty has heretofore been encountered in endeavoringto maintain the desired equilibrium. As decarburization is effected,some of the water vapor in the incoming gas will be consumed, theconsumption varying with the rate of the reaction. If insufficient watervapor is present to off-set the quantity consumed, the decarburizationprocess is slowed for want of the decarburizing reactant. On the otherhand, if an excessive amount of water vapor is present, the excessmoisture will cause scaling which again slows or stops decarburizationuntil the scale is reduced. In addition, such reduced scale is anundesirable contaminant for future processing, such as enameling.

Prior to the instant invention, the customary practice for establishingthe desired dew point in the furnace was to maintain the input dew pointat a constant level. That is, during the decarburizing portion of theanneal the dew point of the incoming gas was adjusted to the equilibriumlevel either by admixing water vapor with the gas prior to itsintroduction into the furnace or by admitting steam along with the gas.However, the difficulty with such prior practice lies in the manyvariable factors which affect the rate of decarburization and hence theconsumption of the water vapor. For example, such factors as the rate atwhich the carbon in the steel diffuses to the surface where it can reactwith the water vapor, the rapidity at which the reaction between thecarbon and the water vapor takes place, together with the rate at whichthe moisture laden gas is brought into contact with the surface of thesteel, all introduce variables which affect the rate of decarburization.Since ideally, the maximum rate of decarburization will occur underconditions wherein the optimum amount of water vapor is available forreaction no matter how fast or slow the decarburization may beatmosphere introduced in the furnace during 3,127,289 Patented Mar. 31,1964 ice 2 taking place and yet insufficient water vapor is present tocause scaling, the prior art practice of maintaining the input dew pointat the equilibrium level does not assure the maintenance of suchconditions in the annealing furnace due to the numerous variables whichare encountered and which cannot be accurately predicted.

Accordingly, a principal object of the instant invention is theprovision of an improved procedure by means of which the H /H Oequilibrium of the atmosphere in the furnace is automaticallymaintained, thereby permitting decarburization to take place at themaximum rate no matter how fast or slow the reaction may be proceedingat any given time during the decarburizing cycle.

A further object of the instant invention contemplates the control ofthe atmosphere in the annealing furnace during decarburization bymaintaining the desired H /H O equilibrium in the exit gas, the hydrogencontent and/or the dew point of the incoming gas being varied so as tomaintain the desired H /H O equilibrium in the exit gas. In other words,the exit H /H O ratio is maintained constant rather than the input fi /HO ratio, as practiced by the prior art.

Still a further object of the instant invention is the provision of aprocess for decarburizing steel in an annealing furnace in a materiallyshortened decarburizing cycle utilizing a single control, namely, dewpoint control of the effluent gas, to maintain essentially optimumdecarburizing conditions without scaling. This control is based upon thediscovery that, if the exit gas is decarburizing and non-oxidizing, thesteel will react accordingly irrespective of inlet gas composition dueto the high recirculation rate of the atmosphere and the high rate ofthe water gas reaction which take place within the furnace.

Still a further object of the instant invention is the provision ofdecarburization-annealing procedures which may be used in conjunctionwith any annealing atmosphere gases which are reducing in nature,inclusive of DX gas, HNX gas, hydrogen, or dissociated ammonia, all ofwhich are readily available and inexpensive.

The foregoing, together with other objects of the instant inventionwhich will appear hereinafter or which will be apparent to the skilledworker in the art upon reading this specification, are accomplished bythose procedures of which exemplary embodiments shall now be described.

Reference is now made to the accompanying drawings wherein:

FIGURE a side elevational view with parts broken away and in section ofa coil heating furnace.

FIGURE 2 is a graph illustrating a conventional decarburizationoperation.

FIGURE 3 is a graph similar to FIGURE 2 but illustrating thedecarburizing operation of the instant invention.

FIGURE 4 is a graph illustrating the dependence of the H2/H2O ratio uponfurnace temperature.

Referring first to FIGURE 4, the graph plots H /H O equilibriums forvarious annealing temperatures. In order to prevent the steel fromoxidizing, the atmosphere within the furnace must lie to the right ofthe curve, with optimum results being obtained when the Bi /H O ratiolies on the curve. Thus, Where the decarburization portion of the annealis conducted at a temperature of 1250 F., optimum results will beachieved when the ratio of hydrogen to water vapor is 2.5:1. For a givenatmosphere having a known hydrogen content, such equilibrium ratio maybe initially established by elevating the dew point of the atmosphere toa predetermined value, which may be on the order of +l30 F.

In accordance with prior art practices, and as illustrated the graphshown in FIGURE 2, the moisture laden decarburiza- 2% tion would bemaintained at a constant dew point !+130 F. in the example given) and itwill be noted that throughout the decarburizing period the dew point ofthe efiluent gas is substantially below that of tron process will beslowed for reactant.

In accordance with the instant invention, it has been discovered thatthe H /H O ratio of the exit gas acconstant reducing potential atsubstantially equilibrium conditions. It is believed that the input gas,so as to thereby maintain equilibrium conditions in the efiluent gas.However, such controls are relatively complex due to the number ofvariables which must be calibrated. Consequently, while it is possibleto control the H /H O ratio of the eflluent gases, it has also beendiscovered that the desired the amount of carbon being removed. However,a typical peak value for input dew point would be 1+160 F. whencontrolling the exit dew point at +130 F.

Essentially, the instant invention may be practiced in conjunction withknown decarburizing and annealing techniques. Thus, the operation beginsby the open winding of the steel strip into coils. Such open or loosewinding contemplates the insertion of a spacer, which may comprisestring, a metal tape, or a twisted Wire, between each convolution of thecoil. This produces a uniform space between each convolution, therebypermitting free passage of the gas between the all over contact of thegas with the entire surface areas of the loose coiled strip. Commercialcoiling units are avai able for this purpose.

Subsequent to the formation of the opened coil, it i transferred to theannealing base of an annealing furnace cover or furnace is vided withsuitable being pro- 5. A gas inlet is prosteam will be injected into theincoming gas as the moisture content of the efiiuent gas drops below thepredetermined level; and similarly, should the dew point of the efiluentgas rise above the desired level, the dew meter will act to reduce orshut off the steam flow until the desired etfiuent dew point has beenreestablished.

constantly controlled annealing at- In other words, one of thereactants, namely, water vapor, is supplied at a rate equal to itsconsumption and demand in the decarburizing re- In addition toincreasing productivity, the constant control of the exit dew point actsto prevent oxidation of the metal. Yet it has been found thatexceedingly high initial relatively dry gas already present in thefurnace.

The instant procedure contemplates the use of any annealing atmospheregases which are reducing in nature, such as the aforementioned DX, HNX,dissociated ammonia or hydrogen gases and mixtures of the same. maximumefiluent gas dew point is limited only by the H /H O equilibrium at thedesired annealing temperature in order to prevent scaling. As apractical matter, the maximum useful dew point will be about +l60 F. andthe minimum useful dew point will be about +70 F. The desired annealingtemperature will be determined by the type of steel being treated; butin general the temperature range will be from about 1100 to 1500 F. Byand large, it is preferred to decarburize at a temperature of about 1250to 1350 F. Hydrogen may be added to the common bright annealing gasesduring the soak period of the annealing cycle in order to increase theallowable dew point, thereby further increasing the decarburizingefiiciency.

Having thus described the invention in certain eXemplary embodiments,and with the understanding that modifications may be made thereinwithout departing from its spirit and purpose, What is desired to secureand protect by Letters Patent is:

1. The method of controlling the hydrogen/Water vapor volume ratio of ahydrogen containing reducing atmosphere in an annealing furnace duringthe decarburization of steel coil, which methodcomprises maintaining theatmosphere in the furnace at a temperature of from about 1100 F. to 1500F. during decarburization, and varying the dew point of the atmosphereas it is introduced into the furnace to the extent necessary to maintainthe dew point of the effluent atmosphere at an essentially constantlevel, at which level the hydrogen/ water vapor volume ratio of theefiiuent atmosphere is within the range of 2.021 to 3.5 :1, thehydrogen/water vapor volume ratio at any given temperature being sochosen that the effluent atmosphere is at all times decarburizing andnon-oxidizing to the steel coil being treated.

2. The method claimed in claim 1 wherein the dew point of the effluentatmosphere is maintained at the said essentially constant level untilthe carbon monoxide content of the effluent atmosphere due to the majordecarburization reaction C+H O=CO+H has fallen below a value on theorder of about one percent.

3. A method of decarburizating steel coil in a con tinuously circulatinghydrogen containing reducing atmosphere having a controlledhydrogen/water vapor volume ratio, which method comprises maintainingthe circulating atmosphere in the furnace at a temperature of from about1100 F. to 1500 F. during decarburization, and maintaining thehydrogen/water vapor volume ratio of the efiiuent atmosphere at anessentially constant level by varying the quantity of moistureintroduced into the entering atmosphere, the quantity of moistureintroduced into the entering atmosphere being the amount necessary tomaintain the dew point of the efiiuent atmosphere at from about +70 F.to about +l F., the dew point at any given temperature being so chosenthat the efiluent atmosphere is at all times decarburizing andnon-oxidizing to the steel coil being treated.

4. The method claimed in claim 3 wherein the dew point of the efliuentatmosphere is maintained at the chosen essentially constant level untilthe carbon monoxide content of the effluent atmosphere due to the majordecarburization reaction C+H O=CO+H has fallen below a value on theorder of about one percent.

References Cited in the file of this patent UNITED STATES PATENTS2,287,467 Carpenter et al. June 23, 1942 2,557,379 Hancock et al June19, 1951 2,753,170 Ness et al. July 3, 1956 3,021,237 Henke Feb. 13,1962 3,030,240 Crombie et al. Apr. 17, 1962

1. THE METHOD OF CONTROLLING THE HYDROGEN/WATER VAPOR VOLUME RATIO OF AHYDROGEN CONTAINING REDUCING ATMOSPHERE IN AN ANNEALING FURNACE DURINGTHE DECARBURIZATION OF STEEL COIL, WHICH METHOD COMPRISES MAINTAININGTHE ATMOSPHERE IN THE FURNACE OF A TEMPERATURE OF FROM ABOUT 1100*F. TO1500*F. DURING DECARBURIZATION, AND VARYING THE DEW POINT OF THEATMOSPHERE AS IT IS INTRODUCED INTO THE FURNACE TO THE EXTENT NECESSARYTO MAINTAIN THE DEW POINT OF THE EFFLUENT ATMOSPHERE AT AN ESSENTIALLYCONSTANT LEVEL, AT WHICH LEVEL THE HYDROGEN/ WATER VAPOR VOLUME RATIO OFTHE EFFLUENT ATMOSPHERE IS WITHIN THE RANGE OF 2.0:1 TO 3.5:1, THEHYDROGEN/WATER VAPOR VOLUME RATIO AT ANY GIVEN TEMPERATURE BEING SOCHOSEN THAT THE EFFLUENT ATMOSPHERE IS AT ALL TIMES DECARBURIZING ANDNON-OXIDIZING TO THE STEEL COIL BEING TREATED.